The making available of this strong material is in response to a pressing demand for a reduction of greenhouse gas emissions, increasingly stringent automobile safety requirements and the price of fuel. These three constraints have pushed the designers and manufacturers of land motor vehicles to use steels with increasingly higher mechanical strength in the body to reduce the thickness of the parts and thus the weight of the vehicles while preserving and even improving the mechanical strength of the structures. Center pillars, bumper crossbars, anti-intrusion parts and other safety parts are examples of parts that require high mechanical strength to perform their primary function and sufficient formability for the shaping of the part in question.
The shaping of steels with a high level of mechanical strength requires a well-known sequence consisting of the genesis of a parent phase austenite, the transformation of the austenite into bainite and/or martensite and optionally the adjustment of the mechanical properties and in particular the hardness of the latter by various heat or thermo-mechanical treatments, depending on the intended functional behavior of the final part.
The mechanical behavior of the martensite is related in particular to the carbon content. The greater the amount of carbon in the martensite, the harder the martensite will be.
The article entitled “Martensite in steel: Strength and structure” by G. Krauss, published in “Materials Science and Engineering” A273-275 (1999), pages 40 to 57, illustrates the link between the carbon content and the hardness of the martensite, this relationship being quasi-linear with the square root of the carbon content in percent by weight. Mechanical strengths significantly greater than 1500 MPa can be achieved by the combination of an increase in the carbon content and the addition of different elements that promote solid solution hardening or precipitation hardening. However, the ductility of a material that has such high strength is prohibitive when it comes to forming a structural part, so that the currently known optimum combination consists of obtaining the high strength level after the forming of the part, via a forming process that can be done hot. It is highly advantageous to have a low strength before the forming and thus an improved ductility to facilitate the forming.
The approach mentioned above is described in patent application WO2009145563 which relates to very high-strength steel sheet that has excellent heat treatment properties, whereby this sheet contains, in % by weight, C: 0.2 to 0.5%, Si: 0.01 to 1.5%, Mn: 0.5 to 2.0%, P: 0.1% or less (but not 0%), S: 0.03% or less (but not 0%), soluble Al: 0.1% or less (but not 0%), N: 0.01 to 0.1%, and Cr: 0.1% to 2.0%, the remainder consisting of iron and unavoidable impurities. This steel sheet has a tensile strength, measured before hot forming, less than or equal to 800 MPa. The sheet is hot formed and rapidly cooled so that it has a tensile strength greater than or equal to 1800 MPa.
However, the levels of carbon described in this document (0.2% to 0.5%) are currently known to be the source of problems in terms of spot welding for the body-in-white of land motor vehicles, i.e. the assembled structure.
Patent application WO200136699 further relates to a composition and a fabrication method for precipitation hardened martensitic stainless steel products, the composition of which contains at least 0.5% by weight Cr and at least 0.5% by weight Mo, whereby the sum of Cr, Ni and Fe exceeds 50%. The microstructure obtained contains at least 50% martensite and the steel is then subjected to an aging treatment between 425 and 525° C. to obtain a precipitation of quasi-crystalline particles. This material meets the requirements of corrosion resistance, high strength and good toughness. The example of the invention is a steel that has an elastic limit of 1820 MPa and a total elongation of 6.7%. If the material obtained is very strong, with a mechanical strength in the range of 1800 MPa, a complex part cannot be formed with such a high-strength sheet, because it is known that the necessary corollary of high strength is relatively low ductility, which leaves little room for maneuver for parts that require formability.